Troubleshooting HVAC Capacitors | Electrical Systems

Troubleshooting HVAC Capacitors - As a technician, it is not uncommon to find bad capacitors. Sometimes it is the primary reason for a trouble call from a customer. The capacitor in the air conditioner or heating system is bad. A good technician can usually figure out the problem very fast. Simply by listening to the symptoms. And what the unit is doing in the sequence of operation. The air conditioner sequence of operation, the gas furnace sequence of operation or the heat pump sequence of operation. Typically the order of sequence of operation of any equipment it is possible a motor is not working properly. This leads the technician to the problem.

Troubleshooting HVAC Capacitors - Capacitor Check

Checking to make sure the capacitor is the problem is often easy for a trained technician. Bad capacitors can often be identified by looking at the capacitor. Often when it goes bad it swells up the outer casing. Sometimes the top blows off leaving an oily residue in the control panel. Sometimes this is not the case, and the capacitor needs to be tested. Tested with a special multimeter that has the function and ability to measure microfarads.

Only a trained HVAC technician or electrician should attempt to troubleshoot a capacitor. There is very high voltage in the cabinet where the capacitor is located. The capacitor also has very high voltage itself. Even with power off and the capacitor disconnected from the equipment and any wires, the capacitor can severely shock you. A capacitor stores electrical power like a battery and will shock you if the proper safety procedures are not followed.

You approach a condenser and you hear a hum. Additionally, the fan is not turning or it’s slowly turning. Definitely not turning at the full RPM’s as it should. You think something is wrong with the motor. You could be right but most likely it is the fan motor capacitor. That is only the case if it is a PSC fan motor. If it is an ECM condenser fan motor then you very likely have another problem altogether. So you follow all safety procedures and later you have the dual run capacitor in your hand. It looks like a pregnant moose compared to the capacitor you know it should be.

If you are lucky it didn’t burst open as some capacitors will do when they go bad. When they burst open they spray oil all inside the cabinet. The oil that is inside the capacitor can smell pretty bad especially coming from a burnt capacitor. At least to those sensitive to bad odors. You will know the capacitor is bad when it is swelled up or it has burst open. If you find a normal looking capacitor then you know you need to test the capacitor. Testing ensures the capacitor is not bad and within range of the rating.

Troubleshooting HVAC Capacitors - Safety and Testing

A dual run capacitor next to a start capacitor with a potential relay above the run capacitor

Before testing a capacitor the capacitor needs to be discharged. Capacitors hold an electrical charge and can shock you even when they are not hooked up to any electrical wiring. The capacitor is discharged using a high resistance resistor (over 10,000 Ohms). This is very important to do as if it is not done the test instrument can be damaged. Once it has been discharged the capacitor can be tested. The instrument measures microfarads which are how capacitors are rated. The test should fall within plus or minus 10% of the rating on the capacitor. When the capacitor is not in the +/- 10% range then it needs to be replaced. Replaced with a new capacitor of the same microfarad rating.

After testing the capacitor you determine it is bad. You go to your truck looking for right microfarad ratings for the capacitor you need to replace. You know you have several new capacitors on the truck. However, you are unsure if you have the correct microfarad ratings for the dual capacitor on your truck. It’s good to keep a good stock of capacitors on the truck. It saves time running back and forth to the supply house.

Troubleshooting HVAC Capacitors - The Best Replacement is an Exact Replacement

The basic policy for replacing electrical parts is that the best replacement part is an exact replacement. You want to stay with what you have and that is what the microfarad rating is on the dual capacitor. If you don’t have a dual but two singles with the proper microfarad ratings you can use those capacitors.

Just make sure you properly mount the capacitors inside the cabinet. Ensure they do not become a hazard and create a dead short inside the cabinet. Carrying band iron is very useful for that purpose. Use your drill and a couple of small set tapping screws and you are good to go. Also, make sure you do not drill through any components or refrigeration piping/coils when mounting capacitors.

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Troubleshooting HVAC Capacitors - Caution Advisory/Example

Warning! When adding self-tapping screws make sure you don’t drill into something like a part of the refrigeration circuit. I’ve seen it happen before so be careful. Using two singles in place of a dual is easy. Let’s say the capacitor that is bad is a dual 45/5 microfarad capacitor. You don’t have a dual 45/5 microfarad capacitor. However, you do have a single 45 microfarad capacitor and a single 5 microfarad capacitor.

You’ll have to jumper the common wire because now you have two capacitors. Whereas before you only had one with one terminal for the common wire. Easy enough for a good HVAC tech to figure that one out. However, I still wanted to be as descriptive as possible for those still learning the trade. It never hurts to go MacGyver to save a little time and be more efficient.

“The two basic types of motor capacitors most commonly used in HVAC applications today. The run capacitor and the start capacitor. Motor capacitors increase the run efficiency of the motor. Capacitors also give the motor an initial boost in torque to a motor upon start-up. HVAC Start Capacitors are referred to as motor starter capacitors or power capacitors.”

Troubleshooting HVAC Capacitors - Parallel Capacitors

If you have a capacitor that keeps failing because of heat issues you can parallel two capacitors to solve the problem. Example: You have a 20 microfarad capacitor that keeps failing because of heat issues. The solution is to take (2) ten microfarad capacitors wired in parallel. This creates more surface area inside the two capacitors (versus using only one 20 microfarad capacitor). More surface area dissipates the heat more efficiently. This is also another solution in case you don’t have a single capacitor for the rating needed. Simply wire two capacitors in parallel that add up to the rating you do need.

A bad dual-run capacitor. The top of the capacitor is all swelled up rather than being flat.

Finally, the last tip to help you with capacitors. If you have a capacitor that has the same microfarad rating but the voltage is different you can use the new capacitor with a different voltage rating only if the voltage rating is higher than the one you are replacing. Again, that is if you are in a pinch. The best replacement for any bad part is an exact replacement.

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